2 things about this:
1. Even if SE did give the current level 50 version of Black Mage ALL the Elemental Spells, you would not be happy because in order to balance the gameplay, Black Mages nukes would have to be very weak, have no enfeebling effects and would probably result in a loss of the 'Ancient Magic' Tier spells for the moment as there would be too many spells for the amount of skills/spells/abilities we can currently have.
2. You said yourself: Black Mage= 'Mage Damage Dealer' So why do they HAVE to have the Elemental Magic? Damage spells are not limited to 'elemental'.
The same logic could be applied to those of us asking you 'Why take these specific from CNJ/WHM to fix this 'boring' job' The reality is: 'Thunder' is just game-code. You are not shooting actual lightning bolts out your wrists killing real-life dragons. The devs could make the spell look like a pink pansy shooting out the black mages butt if they wanted to...so the 'Stone' spell or 'Aero' spell themselves will NOT fix a job you think is broken. That requires reworking the actual job and it's skillset. This can be effectively done WITHOUT stealing skills from other roles. There is no reason why BLM can't have 'Umbral Spike' etc.
..except what you are wanting to do is exactly that overhauling a functional job, to make a boring job more fun and exciting And if that was your goal with your Original Post all along, an effective title would have been something like 'Ideas for Making BLM more exciting' or 'Can Blackmage be more than a one-trick pony in ARR?' or 'New spell options for BLM to encourage more engaged gameplay'. and not 'Hey lets take spells from that pretty mage overthere...that'll make everything better'.
Also, lets just say that SE DID change the Mages spell tree so that BLM did aquire Stone, Stonera, Aero and Aerora. What 4 Spells/abilities should Black Mage/THM give up to gain these spells? ( you'd need to give up 6 if you expect their 'Ancient' counterparts as well) Because every job/class needs to be balanced in how many abilities/spells/skills they have.
Again, we're only MIDGAME level... if you notice, yes, White Mage CAN cast 2 elemental magics...Stone and Aero. But they CANNOT cast the 'Ancient Magic' counterpart. More than likely, Black Mage will get Quake, Flood and Tornado as 'Ancient Magics' and WHM will unlock bigger heals and restorative spells once level caps are raised.